Industry standards for AC and high power DC capacitors have traditionally centered around oil filled capacitor technology. This technology offers benefits of high corona resistance and transient capabilities. Capacitors using this type of technology, however, have problems of potential oil rupture, expensive housings and terminals, poor high frequency response (noisy), mounting restrictions and increased weight. Oil fill technology traditionally employs series disconnects which remove the capacitor from the circuit (or system) by physical distortion of the capacitor housing to break the conductor. These capacitors are permanently disconnected from the circuit and cannot be reset.
Dry film technology offers advantages over oil fill technology. These advantages include broad frequency range, low power loss, light weight, and self healing devices without liquid rupture potential or mounting restrictions. Dry film capacitors, however, have a failure mode that is typically not found in oil fill capacitors. This failure mode is caused by the quality of the capacitor and its electrode configuration which does not allow the capacitor to go to a low resistance short. Instead, the capacitor continually self heals, as the operating temperature inside the capacitor is increased above its operational limits. As the healing continues, the capacitor continues to function and becomes hotter. This, in turn, causes further healing and leads to an avalanching affect. Eventually the capacitor goes to a high resistance short of several ohms, which acts similarly to a heater inside the capacitor and leads to thermal runaway and to gas release due to decomposition of its polymer material and electrode. The onset of these conditions may arise from misapplication of the capacitor, end of life of the capacitor, or premature failure of the capacitor. Failures under these conditions are usually catastrophic and result in hundreds of thousands of dollars in damage to a system and extended off-line periods for repair.
A standard capacitor using dry film technology is the wound capacitor. Wound capacitors are constructed by sandwiching a dielectric film such as polycarbonate, polypropylene or polyester film, between metal electrodes (e.g., vapor deposited metal film). Once formed, the combination dielectric/metal material is wound to form a capacitor. Some specific examples of wound capacitors are found in the following: U.S. Pat. No. 4,719,539 (Lavene), U.S. Pat. No. 4,685,026 (Lavene), and U.S. Pat. No. 5,614,111 (Lavene). Each of these U.S. patents are incorporated herein by reference.
The size of a capacitor is related to its breakdown voltage. The size of a metallized film capacitor is dictated by the thickness of its dielectric film. The thickness of the dielectric, in turn, is dictated by the required overall breakdown voltage of the capacitor. For instance, if a manufacturer cites a particular film as having a dielectric strength of 200 volts/micron and the capacitor design calls for a dielectric breakdown voltage of 400 volts, then the film may be 2 microns thick. Thus, the breakdown voltage of a capacitor depends on the dielectric strength and the thickness of the film.
When electrical current is passed through a wound film capacitor, thermal energy is generated raising the temperature of the capacitor. In large current applications (for example 7 amperes to 30 amperes), this thermal energy can be quite large and may lead to the deterioration of the capacitor. In some applications the thermal energy may even lead to an explosion.
Additionally, thermal energy may be increased if the capacitor is hermetically sealed, because the hermetic sealing may make it more difficult for the heat to be transferred to the exterior of the capacitor and be dissipated. It is known to place metal cover seals at the opposite ends of hermetically sealed capacitors, thereby increasing somewhat the transfer of thermal energy to the exterior of the capacitor. It is also known to provide perforations in these cover seals. The perforations permit outgassing to occur, when the capacitor is baked prior to sealing, thereby cleaning and drying the capacitor.
It is known to provide fault interrupters to prevent capacitors from overheating or exploding. U.S. Pat. No. 3,496,432 discloses a wound capacitor which forms gas when being overheated. The dielectric of the capacitor winding includes a foil of thermoplastic material with the property of contracting when heated. Thus, when the capacitor winding, upon heating, contracts in the axial direction, one of the metal layers is separated from the capacitor winding, so that electrical connection to the capacitor winding is interrupted.
U.S. Pat. No. 4,639,827 discloses a pressure sensitive fault interrupter for a film capacitor. The film capacitor has a dome-shaped diaphragm. When a fault occurs, pressure is developed within the capacitor as a result of the breakdown of the dielectric, thereby producing various gases. These gases fill the core of the capacitor and exert downward pressure on the diaphragm. The downward pressure changes the concave shape of the diaphragm into a convex shape, thereby breaking the electrical contact between the film capacitor and one of its tabs.
In the prior art, a capacitor may have a fault interrupter that permanently disables the capacitor. This protects the system that houses the capacitor. However, the capacitor cannot be reset and cannot be re-used, after the temperature of the capacitor reaches an acceptable level. The present invention, as described below, includes a sensor that senses a predetermined temperature level of the capacitor and provides an external alert to a user. The user may decide whether to continue operation of the capacitor in the system or shut down the system. The sensor is re-settable and the capacitor may be re-used in the system.